Comment 3 for bug 1862460

Sorry, but I'm lost with your long text - and many new things which are (for me) unrelated to the problem.

The mask is copied as text to the clipboard and pasted also as text from the clipboard (the format is the same as the mask file).

So lets do it step by step: create mask.
Then save mask to file (export) and then try to copy to clipboard. Now check the content of the clipboard (either in Finder, Edit>Show clipboard or try to paste in your editor).